Conventionally, a combustion apparatus, such as an internal combustion engine, uses a spark plug for igniting an air-fuel mixture through spark discharge. In recent years, in order to meet demand for high output and low fuel consumption of the combustion apparatus, a plasma jet ignition plug has been proposed, since the plasma jet ignition plug provides quick propagation of combustion and can more reliably ignite even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio.
Generally, the plasma jet ignition plug includes a tubular insulator having an axial bore, a center electrode inserted into the axial bore in such a manner that a forward end surface thereof is recessed from a forward end surface of the insulator, a metallic shell disposed externally of the outer circumference of the insulator, and an annular ground electrode joined to a forward end portion of the metallic shell. Also, the plasma jet ignition plug has a hollow space (cavity) defined by the forward end surface of the center electrode and a wall surface of the axial bore. The cavity communicates with an ambient atmosphere via a through hole formed in the ground electrode.
Such plasma jet ignition plug ignites an air-fuel mixture as follows. First, voltage is applied across a gap formed between the center electrode and the ground electrode, thereby generating spark discharge across the gap and thus causing dielectric breakdown in the gap. In this condition, electric power is input to the gap, thereby bringing gas in the cavity into the plasma state; i.e., generating plasma within the cavity. The generated plasma is discharged through an opening of the cavity, thereby igniting the air-fuel mixture.
In order to generate plasma in the plasma jet ignition plug, a general ignition system for the plasma jet ignition plug includes a voltage application section for applying voltage across the gap to generate spark discharge, and an electric power input section for inputting electric power to the gap. Also, diodes are provided respectively between the plasma jet ignition plug and the voltage application section and between the plasma jet ignition plug and the electric power input section, so as to prevent flow of current from one of the voltage application section and the electric power input section to the other. For example, refer to Japanese Patent Application Laid-Open (kokai) No. 2010-218768 (“Patent Document 1”).